U.S. patent number 5,270,009 [Application Number 07/942,407] was granted by the patent office on 1993-12-14 for method of and apparatus for measuring oxygen concentration with limiting current oxygen sensor.
This patent grant is currently assigned to Shimadzu Corporation. Invention is credited to Masaru Kozakura, Akioki Nakamori.
United States Patent |
5,270,009 |
Nakamori , et al. |
December 14, 1993 |
Method of and apparatus for measuring oxygen concentration with
limiting current oxygen sensor
Abstract
Relation of the degree of interference to interference
concentration is previously stored with respect to at least one
interference gas influencing an oxygen indicated value of a sample
gas, so that interference corresponding to interference gas
concentration of the sample gas is corrected on the basis of the
stored relation when oxygen concentration is calculated from a
detection signal of an oxygen sensor and outputted. Concentration
of the interference gas, with respect to which the relation of the
degree of interference is stored, is manually inputted in
measurement, or previously set. When the concentration of the
interference gas is previously set, such concentration is made
changeable by manual input. Thus, even if the sample gas contains
an interference gas in high concentration as in the case of an
exhaust gas containing highly concentrated CO.sub.2 gas, it is
possible to accurately measure oxygen concentration of the sample
gas.
Inventors: |
Nakamori; Akioki (Kyoto,
JP), Kozakura; Masaru (Kyoto, JP) |
Assignee: |
Shimadzu Corporation (Kyoto,
JP)
|
Family
ID: |
17572548 |
Appl.
No.: |
07/942,407 |
Filed: |
September 9, 1992 |
Foreign Application Priority Data
|
|
|
|
|
Sep 27, 1991 [JP] |
|
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3-276660 |
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Current U.S.
Class: |
422/83; 422/98;
73/1.07; 73/23.21; 73/23.31; 73/31.01 |
Current CPC
Class: |
G01N
27/4065 (20130101) |
Current International
Class: |
G01N
27/406 (20060101); G01N 027/00 () |
Field of
Search: |
;422/83,98
;73/23.21,23.31,1G,31.01 ;364/571.01,556,551.01 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Warden; Robert J.
Assistant Examiner: Tran; Hien
Attorney, Agent or Firm: Klima; William L.
Claims
What is claimed is:
1. A limiting current oxygen concentration measuring apparatus,
comprising:
a sample gas supply;
a calibration gas supply;
an oxygen concentration detector formed by a limiting current
oxygen sensor;
a gas switching valve connected between said sample gas and
calibration gas supplies and said oxygen concentration detector for
selectively switching calibration gas and sample gas containing at
least one interference gas for introducing the same into said
oxygen concentration detector;
a memory unit storing a relation of degree of interference to
interference gas concentration of at least one interference gas
influencing an oxygen indicated value of said sample gas; and
an arithmetic processing part for correcting interference
corresponding to concentration of said interference gas with
respect to a detection value of said oxygen concentration detector
on a basis of said relation stored in said memory unit.
2. An apparatus in accordance with claim 1, further comprising an
input unit for manually inputting concentration of said
interference gas.
3. An apparatus in accordance with claim 1, wherein
concentration of said interference gas is previously set in said
arithmetic processing part,
said apparatus further comprising an input unit for inputting said
concentration of said interference gas, so that interference gas
concentration is manually inputted from said input unit for
changing said concentration when concentration of said interference
gas contained in said sample gas is different from said previously
set concentration.
4. An apparatus in accordance with claim 1, wherein said arithmetic
processing part is connected to said gas switching valve for
selectively controlling the introduction of sample and calibration
gases.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of and an apparatus for
measuring oxygen concentration in a sample gas with a limiting
current oxygen sensor, and more particularly, it relates to
measuring method and apparatus which are suitable for measuring
oxygen concentration in a sample gas, such as an exhaust gas, for
example, containing a great deal of an interference gas.
2. Description of the Background Art
A limiting current oxygen concentration measuring apparatus is
provided with a limiting current oxygen sensor comprising
electrodes which are formed on both sides of a stabilized zirconia
member. When a voltage is applied across the cathode and the anode
of the oxygen sensor during bringing a sample gas into contact with
the cathode, a current flows through carriers of oxygen ions
contained in the sample gas by pumping action of the stabilized
zirconia member. Thus, oxygen contained in the sample gas is
discharged toward the anode through the stabilized zirconia member.
At this time, the current becomes constant with respect to the
oxygen concentration of the sample gas in a certain range of the
voltage across the electrodes. Such a constant current is called a
limiting current. Since the oxygen concentration has linear
relation to the limiting current, it is possible to detect the
oxygen concentration by measuring the value of the limiting
current.
A limiting current oxygen sensor employing such a principle is
generally adapted to measure oxygen concentration in a clean gas,
or a gas which is approximate in composition to the atmospheric
gas. When the sample gas is an exhaust gas from an automobile or a
factory and contains CO.sub.2 in high concentration, however,
CO.sub.2 serves as an interference gas for oxygen in the oxygen
sensor to hinder correct measurement of oxygen concentration. In
general, therefore, the oxygen sensor is not employed for
measurement of a gas such as an exhaust gas containing CO.sub.2 in
high concentration.
When CO.sub.2 contained in a sample gas having high CO.sub.2
concentration is absorbed by an absorbent in order to measure
oxygen concentration with the oxygen sensor, the composition of the
sample gas is so changed that oxygen concentration cannot be
correctly attained in a sampling point.
In the aforementioned oxygen sensor, further, it is necessary to
limit the inflow of oxygen molecules by some means. When the inflow
is limited by a small hole, for example, no problem is caused in
measurement of a gas which is substantially unchanged in spun gas
composition. If a gas containing a great deal of foreign gas
molecules, such as those of CO.sub.2 contained in an exhaust gas,
in percentage order, however, physical properties of the sample gas
are so changed that an indicated value of oxygen concentration
cannot be correctly attained.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and an
apparatus which can accurately measure oxygen concentration in a
sample gas with a limiting current oxygen sensor, even if the
sample gas contains an interference gas, such as highly
concentrated CO.sub.2 gas contained in an exhaust gas, in high
concentration.
According to the inventive method of measuring oxygen
concentration, the degree of relation of interference to
interference gas concentration is previously stored with respect to
at least one interference gas influencing an oxygen indicated value
of a sample gas, so that interference corresponding to interference
gas concentration of the sample gas is corrected on the basis of
the stored relation when oxygen concentration is calculated from a
detection signal of an oxygen sensor and outputted. Concentration
of the interference gas, with respect to which the degree of
relation of interference is stored, is manually inputted in
measurement, or previously set. When the concentration of the
interference gas is previously set, the same is made changeable by
manual input.
On the other hand, the inventive apparatus for measuring oxygen
concentration comprises an oxygen concentration detector formed by
a limiting current oxygen sensor, a gas inlet path having an inflow
gas switching valve for switching constant volumes of a calibration
gas and sample gas for introducing the same into the oxygen
concentration detector, a memory unit storing relation of the
degree of interference to interference gas concentration with
respect to at least one interference gas influencing an oxygen
indicated value of the sample gas, and an arithmetic processing
part for correcting interference corresponding to concentration of
the interference gas contained in the sample gas with respect to a
detected value of the oxygen concentration detector on the basis of
the relation stored in the memory unit.
According to a preferred aspect of the present invention, the
apparatus for measuring oxygen concentration further comprises an
input unit, so that concentration of the interference gas can be
manually inputted.
In one of the fields to which the inventive method and apparatus
are applied, oxygen concentration of an exhaust gas from an
automobile or a factory is measured. Such an exhaust gas contains
CO.sub.2 gas as the interference gas. FIG. 2 shows interference of
CO.sub.2 against oxygen concentration which is detected by a
limiting current oxygen sensor, in the form of deviation rates (%)
with respect to indicated values of oxygen concentration to
CO.sub.2 gas concentration (volume percent throughout the
specification). Interference of CO.sub.2 results in indicated
values of oxygen concentration which are lower than actual values.
Since it is known that an exhaust gas contains about 5 to 15% of
CO.sub.2 gas, CO.sub.2 gas concentration is set at 10%, in order to
measure such an exhaust gas. Thus, detected values of oxygen
concentration are shifted by 1.25% toward higher sides, to be
outputted. It is understood that interference of CO.sub.2 in oxygen
sensor can be suppressed within about .+-.1% by this simple
correction method, if CO.sub.2 gas concentration is in a range of 5
to 15%.
If the sample gas contains CO.sub.2 gas in concentration of 20%, it
is possible to correct indicated values of oxygen concentration by
re-setting CO.sub.2 concentration at 20% through the input
unit.
In order to apply the inventive apparatus for measuring oxygen
concentration to an exhaust gas measuring apparatus, CO.sub.2 gas
concentration may be previously set at 10%, so that this CO.sub.2
gas concentration is re-set through the input unit only when a gas
containing CO.sub.2 gas in concentration being extremely out of 10%
is measured.
The foregoing and other objects, features, aspects and advantages
of the present invention will become more apparent from the
following detailed description of the present invention when taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a limiting current oxygen
concentration measuring apparatus according to the present
invention;
FIG. 2 is a graph expressing CO.sub.2 interference in relation to
an oxygen sensor, in the form of deviation rates (%) of oxygen
concentration indicated values with respect to CO.sub.2 gas
concentration before and after correction;
FIG. 3 is a flow chart showing the procedure of measuring oxygen
concentration in a gas containing CO.sub.2 gas with the inventive
apparatus; and
FIG. 4 is an end view schematically showing the principle of a
limiting current oxygen sensor employed in an embodiment of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a block diagram showing the structure of a limiting
current oxygen concentration measuring apparatus according to the
present invention. Referring to FIG. 1, a sample gas inlet path 1
and a calibration gas inlet path 2 are connected to an inflow gas
switching valve 3, which in turn switches a sample gas and a
calibration gas and guides the same to an oxygen concentration
detector 4 which is formed by a limiting current oxygen sensor. The
sample gas inlet path 1, the calibration gas inlet path 2 and the
switching valve 3 define a gas inlet path. A memory unit 7 is
adapted to store relation between interference gas concentration
and the degree of interference with respect to an interference gas
which influences an oxygen concentration indicated value of the
sample gas. An arithmetic processing part 5 including a CPU
(Central Processing Unit) is adapted to control the operation of
the switching valve 3, as well as to output on oxygen concentration
value of the sample gas to an output unit 6 by a concentration
signal from the detector 4. It is indicated that the sample gas
contains an interference gas, the arithmetic processing part 5
outputs a corrected oxygen concentration value to the output unit 6
using a correction system as described later and the relation of
interference stored in the memory unit 7. The output unit 6 is
formed by a display unit such as a CRT (cathode ray table), a
printer, or a recorder. An input unit 8 such as a keyboard is
adapted to set CO.sub.2 concentration of the sample gas with
respect to the arithmetic processing part 5 and change the
same.
FIG. 4 shows the limiting current oxygen sensor which is applied to
the oxygen concentration detector 4. Referring to FIG. 4, the
limiting current oxygen sensor comprises a solid electrolytic
member 10 of zirconia (ZrO.sub.2) which is stabilized by a solid
solution of Y.sub.2 O.sub.3 or CaO, platinum paste electrodes 11
and 12 which are formed on both sides of the solid electrolytic
member 10, and a cap 13, having a small hole 13a, which is
connected to one side of the solid electrolytic member 10 provided
with the electrode 12. When a voltage is applied from a power
source 14 so that the electrode 12 serves as a cathode and the
other electrode 11 serves as an anode, a current flows through
carriers of oxygen ions by pumping action of the solid electrolytic
member 10, to discharge oxygen contained in the cap 13 from the
electrode 12 toward the other electrode 11. The applied voltage
from the power source 14 is so adjusted that the current flowing in
the solid electrolytic member 10 reaches a limiting current value
which has linear relation to oxygen concentration in the cap
13.
With reference to FIG. 3, description is now made on the procedure
of detecting and correcting oxygen concentration in a sample gas
with the oxygen concentration measuring apparatus shown in FIG.
1.
(1) It is inputted that the inlet paths 1 and 2 are for sample and
calibration gases respectively, so that constant volumes of sample
and calibration gases are introduced into the switching valve 3
from the sample and calibration gas inlet paths 1 and 2
respectively, and a start switch is turned on.
(2) The arithmetic processing part 5 controls the switching valve
3, and makes a determination as to whether or not the inflow gas is
the calibration gas, depending on whether the switching valve 3 is
switched toward the sample gas or the calibration gas. If the
inflow gas is sample gas, a determination is made as to whether or
not CO.sub.2 gas is contained. This determination is made along
previous setting or manual input made by a measurer through the
input unit 8.
(3) Assuming the CO.sub.2 concentration in the sample gas is set at
10% in the measuring apparatus for measuring an exhaust gas, it is
assumed that CO.sub.2 gas concentration is 10% when a determination
is made that CO.sub.2 gas is contained, unless the measurer inputs
CO.sub.2 concentration other than 10%. If CO.sub.2 gas
concentration other that 10% is inputted, a corrected value
obtained by shifting a deviation value shown in FIG. 2 is outputted
along the inputted CO.sub.2 concentration. The operation for this
correction is made along the following expression:
where KO.sub.2 represents O.sub.2 concentration after correction,
KO.sub.2 ' represents O.sub.2 concentration before correction,
KCO.sub.2 represents CO.sub.2 concentration in the sample gas, and
.alpha. represents the degree of influence of CO.sub.2 interference
which takes the value shown in FIG. 2 by CO.sub.2
concentration.
(4) If the sample gas contains no CO.sub.2 gas, the measured value
is directly outputted with no correction.
(5) If a determination is made that the inflow gas in the switching
valve 3 is the calibration gas, no correction is made but the
switching valve 3 is switched toward the sample gas side by the
arithmetic processing part 5, and the start switch is again turned
on.
If CO.sub.2 gas concentration in the sample gas is known, high
accuracy correction is enabled by carrying out the correction not
with the previously set value of 10% but with CO.sub.2 gas
concentration changed to the known concentration.
Although the arithmetic processing part 5 is formed by a CPU in the
above description, correction may be analogously made by an
electric circuit, in place of such a CPU.
As to a gas, other than CO.sub.2 gas, which is contained in a
sample gas in a great deal to change gas physical properties, it is
possible to make correction similarly to that for CO.sub.2 gas, as
an interference gas influencing measurement of oxygen
concentration, to output a measured oxygen concentration value.
When an oxygen concentration indicated value influenced by an
interference gas such as CO.sub.2 gas is not corrected as in the
prior art, it is impossible to correctly measure oxygen
concentration of a sample gas such as an exhaust gas containing a
great deal of CO.sub.2. According to the present invention, on the
other hand, it is possible to measure oxygen concentration in
relatively high accuracy with simple correction means.
Although the present invention has been described and illustrated
in detail, it is clearly understood that the same is by way of
illustration and example only and is not to be taken by way of
limitation, the spirit and scope of the present invention being
limited only by the terms of the appended claims.
The features disclosed in the foregoing description, in the claims
and/or in the accompanying drawings may, both, separately and in
any combination thereof, be material for realising the invention in
diverse forms thereof.
* * * * *